Handle condition of invalid submit filter more gracefully

While tracking down a very obscure issue with qsub (there's a list of about 15 requirements to reproduce) I found that the PBS_O_SUBMIT_FILTER was being checked for validity but nothing was actually being done with the results of said check. I may file an issue about the obscure issue  if I can get some more data points, but essentially the PBS_O_SUBMIT_FILTER gets lost and ends up as a null string. This produces some mystical errors when trying to submit a job.

This change probably isn't super useful, but this would have given us a head start on where to look for the cause of the mystical errors. I can see the argument that invalid filters may be fairly obvious ("sh: /invalid/submit/filter: not found") on run time but that doesn't seem very graceful. Why bother stat()'ing the filter in validate_submit_filter() if the results aren't being acted on?

This is my first time jumping into torque code so this may not be the best way to code this :)